Device for capacitive measurements in a multi-phase medium

ABSTRACT

A device comprises at least one pair of excitation electrodes forming a capacitor; a floorplan (e.g., a ground plane); and an electronic circuit. The device comprises at least one control electrode arranged at a distance from the capacitor. A switching circuit, of the device, comprises a switch having an open state and a closed state. The switching circuit is designed to apply, to the control electrode, an electric potential common to the floorplan when the switch is in the closed state. The switching circuit is also designed to leave a floating electrical potential for the control electrode when the switch is in the open state. The electronic circuit is designed to measure the mutual capacitance between the pair of excitation electrodes when the switch is in the open state and when it is in the closed state.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase entry under 35 U.S.C. § 371 ofInternational Patent Application PCT/FR2019/050033, filed Jan. 8, 2019,designating the United States of America and published as InternationalPatent Publication WO 2019/138181 A1 on Jul. 18, 2019, which claims thebenefit under Article 8 of the Patent Cooperation Treaty to FrenchPatent Application Serial No. 1850268, filed Jan. 12, 2018.

TECHNICAL FIELD

The present disclosure relates to the technical field of devices fortaking capacitive measurements of a multiphase medium, the multiphasemedium containing fluid phases that differ in their dielectricproperties, for example an aqueous phase and at least one organic phase.

Devices for taking capacitive measurements have the advantage of beingable to operate at a single working frequency, contrary to devices fortaking impedance measurements (e.g., by electrochemical impedancespectroscopy), which require a complete scan of the frequencies inquestion.

The present disclosure is notably applicable to:

-   -   detecting the presence of an aqueous phase within a multiphase        medium containing at least one organic phase (e.g., oil,        kerosene, fuel oil, etc.),    -   determining the height of the aqueous phase in a vessel        containing a multiphase medium containing at least one organic        phase (e.g., oil, kerosene, fuel oil, etc.),    -   determining the height of sedimented or floating layers in a        phase separator (e.g., sludges, fats, etc.), and    -   monitoring drying of a porous multiphase medium (e.g., concrete,        wet earth, etc.).

BACKGROUND

One device for taking capacitive measurements that is known in the priorart, and notably from document U.S. Pat. No. 7,258,005, comprises:

-   -   at least one pair of excitation electrodes, forming a capacitor,        and intended to be inserted into the multiphase medium;    -   a ground plane;    -   an electronic circuit, arranged to electrically connect the pair        of excitation electrodes to the ground plane, and configured to:    -   apply an electrical potential to each excitation electrode at a        working frequency, and measure a transcapacitance between the        pair of excitation electrodes.

This prior-art device is not entirely satisfactory in so far as itrequires a particular geometric arrangement of the excitationelectrodes, and, in particular, the spacing therebetween to be largerthan or equal to two times the thickness of the dielectric walls of avessel containing the multiphase medium (see claim 1; column 5, lines21-25; column 6, lines 4-5). If the spacing between the excitationelectrodes is smaller than two times the thickness of the dielectricwalls of the vessel, the dependence of the value of the transcapacitanceon the height of the fluid decreases (see column 6, lines 7-8) and hencethe sensitivity and accuracy of the measurement decreases.

BRIEF SUMMARY

The present disclosure aims to completely or partially remedy theaforementioned drawbacks. To this end, one subject of the presentdisclosure is a device for taking capacitive measurements in amultiphase medium, comprising:

-   -   at least one pair of excitation electrodes, forming a capacitor;    -   a ground plane; and    -   an electronic circuit, arranged to electrically connect the pair        of excitation electrodes to the ground plane, and configured to:    -   apply an electrical potential to each excitation electrode at a        working frequency, and measure a transcapacitance between the        pair of excitation electrodes;        the device being noteworthy in that it comprises:    -   at least one control electrode, arranged at a distance from the        capacitor, and intended to be inserted into the multiphase        medium; and    -   a switching circuit comprising a switch having an open state and        a closed state, in which states the switch electrically        disconnects and connects the control electrode from/to the        ground plane, respectively, the switching circuit being        configured to:    -   apply, to the control electrode, an electrical potential common        to the ground plane, when the switch is in the closed state, and    -   leave the electrical potential of the control electrode        floating, when the switch is in the open state; and        in that the electronic circuit is configured to measure the        transcapacitance between the pair of excitation electrodes when        the switch is in the open state and when the switch is in the        closed state.

Thus, such a device according to the present disclosure allows, byvirtue of the control electrode, the presence of an electricallyconductive phase to be detected at the working frequency provided thatthe transcapacitance measured when the switch is in the open state isdifferent from the transcapacitance measured when the switch is in theclosed state.

Furthermore, such a device according to the present disclosure allows,by virtue of the control electrode, the height of the detected phase ina vessel to be determined on the basis of the transcapacitances measuredwhen the switch is in the open state and in the closed state.

Lastly, the operation of such a device according to the presentdisclosure places no specific constraints on the spacing betweenexcitation electrodes, contrary to the prior art.

Definitions

By “multiphase medium,” what is meant is a medium containing phases thatdiffer in their dielectric properties. The phases are not necessarilyimmiscible. The multiphase medium may contain fluid phases and/or solidphases, for example, when the multiphase medium is porous.

By “ground plane,” what is meant is any means for obtaining a referencepotential for the device.

By “transcapacitance,” what is meant is the electrical capacitancebetween the pair of excitation electrodes, i.e., the ratio between theamount of electrical charge borne by one excitation electrode and thepotential difference between the two excitation electrodes. Thetranscapacitance therefore differs from the electrical capacitancebetween either of the excitation electrodes and the ground plane.

The device according to the present disclosure may comprise one or moreof the following features.

According to one feature of embodiments of the present disclosure, themultiphase medium comprises a phase containing species that areelectrically conductive at the working frequency; the species possessinga cut-off frequency, below which the species equalize the electricalpotential, left floating, in the multiphase medium, over the distance atwhich is arranged the control electrode of the capacitor; and theworking frequency is chosen so as to be lower than or equal to thecut-off frequency.

By “equalize” what is meant is:

-   -   either a strict equalization of the electrical potential, left        floating, in the multiphase medium, over the distance at which        is arranged the control electrode of the capacitor; or    -   an approximate equalization of the electrical potential, left        floating, in the multiphase medium, over the distance at which        is arranged the control electrode of the capacitor, so that the        potential difference between the electrically conductive phase        (at the working frequency) and the control electrode remains        negligible.

One advantageous effect thereof is to improve the reliability of thedevice in case of detection of phase presence, and to improve theaccuracy of the measurement in case of determination of the height ofthe phase in a vessel, for example.

According to one feature of embodiments of the present disclosure, thecapacitor has a characteristic distance, denoted d; and

-   -   the distance at which is arranged the control electrode of the        capacitor, which distance is denoted l, is chosen so that:

$0 < l < {100 \times d\frac{f_{c}}{f}\mspace{14mu} {and}\mspace{14mu} {preferably}}$$0 < l < {10 \times d\frac{f_{c}}{f}}$

where:

-   -   f_(c) is the cut-off frequency, and    -   f is the working frequency.

Thus, a large distance l (i.e., close to 100 d f_(c)/f) may be chosenprovided that the electronic circuit has a very high level ofperformance in terms of the accuracy of the measurement of thetranscapacitance between the pair of excitation electrodes. Conversely,a smaller distance l (i.e., of as little as 10 d f_(c)/f) will be chosenif the electronic circuit possesses a conventional level of performancein terms of the accuracy of the measurement of the transcapacitancebetween the pair of excitation electrodes.

According to one feature of embodiments of the present disclosure, thedevice comprises a set of control electrodes, these electrodes beingarranged at various distances from the capacitor and being intended tobe inserted into the multiphase medium, the switching circuit comprisingone dedicated switch for each control electrode.

One advantageous effect thereof is to allow, within the multiphasemedium, the presence or amount of the electrically conductive phase tobe studied spatially at the working frequency. By way of example, it isthen possible to monitor the drying state of a porous and moistmultiphase medium.

According to one feature of embodiments of the present disclosure, theelectronic circuit comprises a virtual ground connected to an excitationelectrode, and the electronic circuit is configured to measure thetranscapacitance between the pair of excitation electrodes using athree- or four-wire method.

One advantageous effect of the virtual ground, and of the three- orfour-wire method, is to make it possible to allow for parasiticcapacitances between an excitation electrode and the ground plane, aswell as the impedance of a wire, so as to obtain an accurate measurementof the transcapacitance between the pair of excitation electrodes.

According to one feature of embodiments of the present disclosure, theelectronic circuit comprises an operational amplifier used as aninverter, and comprising:

-   -   a non-inverting input, connected to the ground plane; and    -   an inverting input, connected to an excitation electrode.

One advantageous effect thereof is to make it easy to obtain a virtualground. In other words, such a use allows the excitation electrodeconnected to the inverting input to be placed virtually at groundpotential, when the operational amplifier is employed in the linearregime.

According to one feature of embodiments of the present disclosure, thedevice comprises:

-   -   a dielectric layer, comprising a first surface and an opposite        second surface, the pair of excitation electrodes extending to        the first surface of the dielectric layer; and    -   a counter-electrode, extending to the second surface of the        dielectric layer, and forming the ground plane.

One advantageous effect thereof is to allow the assembly formed by thepair of excitation electrodes, the dielectric layer, and thecounter-electrode to be inserted into the multiphase medium. Thedielectric layer allows the pair of excitation electrodes and thecounter-electrode to be electrically insulated from each another.

According to one feature of embodiments of the present disclosure, thepair of excitation electrodes is covered with a dielectric film.

One advantageous effect thereof is to protect the pair of excitationelectrodes from the multiphase medium.

According to one feature of embodiments of the present disclosure, thecapacitor formed by the pair of excitation electrodes is selected from aparallel-plate capacitor, a capacitor with interdigitated electrodes,and a coaxial-cylinder capacitor.

One advantageous effect of the capacitor with interdigitated electrodesis to inhibit to a lesser extent the movement of the electricallyconductive ions (which is notably inhibited with the parallel-plate andcoaxial-cylinder capacitors), this making it possible to more easilyreach the electrical potential, left floating, in the multiphase medium,over the distance (1) at which is arranged the control electrode of thecapacitor. This floating potential will possibly be obtained using anelectrical system possessing a very high impedance with respect to acharge reservoir, such as that of an operational amplifier.

With parallel-plate capacitors, potential gradients have been observedto form in the multiphase medium if the distance l is increased. Inother words, a capacitor with interdigitated electrodes allows greaterfreedom over the distance at which is arranged the control electrode ofthe capacitor.

Another advantageous effect of a capacitor with interdigitatedelectrodes is to decrease the effects of electrical double layers at thewall (which effects are notably observed with parallel-plate andcoaxial-cylinder capacitors). These effects tend to increase the valueof the transcapacitance when the switch is in the open state.

Another subject of the present disclosure is an installation,comprising:

-   -   a vessel containing a multiphase medium; and    -   a device according to the present disclosure, the control        electrode being inserted into the multiphase medium.

Another subject of the present disclosure is a system for takingcapacitive measurements in a multiphase medium, comprising:

-   -   a floating device, intended to float in the multiphase medium;        and    -   at least one device according to the present disclosure,        securely fastened to the floating device.

Thus, such a system, according to the present disclosure, allows validcapacitive measurements to be taken when the height of the multiphasemedium does not remain constant over time, by allowing a referenceheight to be set relative to the free surface of the multiphase mediumby virtue of the presence of the floating device. It is then possible toaccurately determine the height of sedimented or floating layers in aphase separator (e.g., hydrocarbons, light sludges, fats, etc.) such asa sewage treatment plant (or micro-plant). Specifically, in this type ofapplication, the position of the free surface of the top phase of themultiphase medium may vary over time, because of variations in theliquid (water) flow rate and in the amount of floating hydrocarbons. Thecapacitive measurements delivered by such a system, according to thepresent disclosure, allow at what moment to act to remove a pollutingphase (generally by pumping) to be determined by detecting the pollutingphase. Specifically, it is important to be able to plan this type ofinterventions as they are expensive and missing them may lead topollution being introduced into the environment. Moreover, thecapacitive measurements delivered by such a system, according to thepresent disclosure, allow only the strictly necessary amount ofpolluting phase to be removed, as they allow the height of the pollutingphase to be monitored.

According to one feature of embodiments of the present disclosure, thefloating device comprises a separating wall forming a barrier to themultiphase medium, the wall possessing an internal surface, and thedevice for taking capacitive measurements is mounted inside the wall,against the internal surface.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages will become apparent from the detaileddescription of various embodiments, the description containing examplesand references to the appended drawings.

FIG. 1 is a schematic view illustrating the circuitry of a deviceaccording to the present disclosure.

FIG. 2 is a schematic perspective view, illustrating one embodiment of adevice according to the present disclosure.

FIG. 3 is a schematic perspective view, illustrating one embodiment of adevice according to the present disclosure.

FIG. 4 is a schematic perspective view, illustrating one embodiment of adevice according to the present disclosure.

FIG. 5 is a schematic perspective view, illustrating one embodiment of adevice according to the present disclosure.

FIG. 6 is a schematic cross-sectional view, illustrating one embodimentof a device according to the present disclosure.

FIG. 7 is a graph, the x-axis of which represents the observed oil level(in mm) and the y-axis of which represents the phase level (in mm)determined from the capacitive measurements of a device according to thepresent disclosure. “A” is an aqueous phase, “B” is an oil phase, and“C” is an air phase.

FIG. 8 is a graph, the x-axis of which represents the observed waterlevel (in % of the total height of the multiphase medium) and the y-axisof which represents the phase level (in % of the total height of themultiphase medium) determined from the capacitive measurements of adevice according to the present disclosure. “A” is an aqueous phase, “B”is an oil phase, and “C” is an air phase.

FIGS. 9a to 9e are schematic cross-sectional views, illustrating variousembodiments of such a system according to the present disclosure.

DETAILED DESCRIPTION

For the sake of simplicity, elements that are identical or that performthe same function have been designated with the same references in thevarious embodiments.

One subject of the present disclosure is a device for taking capacitivemeasurements in a multiphase medium M, comprising:

-   -   at least one pair of excitation electrodes 1, 2, forming a        capacitor;    -   a ground plane PM; and    -   an electronic circuit 3, arranged to electrically connect the        pair of excitation electrodes 1, 2 to the ground plane PM, and        configured to:    -   apply an electrical potential V+, V− to each excitation        electrode 1, 2 at a working frequency, and    -   measure a transcapacitance between the pair of excitation        electrodes 1, 2;        the device being noteworthy in that it comprises:    -   at least one control electrode 4, arranged at a distance l from        the capacitor, and intended to be inserted into the multiphase        medium M; and    -   a switching circuit 5 comprising a switch 50 having an open        state and a closed state, in which states the switch 50        electrically disconnects and connects the control electrode 4        from/to the ground plane PM, respectively, the switching circuit        5 being configured to:    -   apply, to the control electrode 4, an electrical potential        common to the ground plane PM, when the switch 50 is in the        closed state, and    -   leave the electrical potential of the control electrode 4        floating, when the switch 50 is in the open state; and        in that the electronic circuit 3 is configured to measure the        transcapacitance between the pair of excitation electrodes 1, 2        when the switch 50 is in the open state and when the switch 50        is in the closed state.

Multiphase Medium

The multiphase medium M contains species forming phases P₁, P₂, P₃. Thespecies possess a cut-off frequency, below which the species equalizethe electrical potential, left floating, in the multiphase medium M,over the distance l at which is arranged the control electrode 4 of thecapacitor. It is assumed that one of the species is electricallyconductive at the working frequency. By way of example, the workingfrequency may be set to 1 kHz to detect the presence of an aqueous phaseP₁. The multiphase medium M sets the electrical potential (left floatingby the switch 50) of the control electrode 4 when the switch 50 is inthe open state.

In the case illustrated in FIG. 6, the multiphase medium M containsthree phases P₁, P₂, P₃ that are stratified, that differ in theirdielectric properties, and that have a height h₁, h₂, h₃ in a vessel Rof height H, respectively.

Capacitor

The capacitor formed by the pair of excitation electrodes 1, 2 isadvantageously selected from a parallel-plate capacitor (as illustratedin FIG. 2), a capacitor with interdigitated electrodes (as illustratedin FIGS. 4 to 6), and a coaxial-cylinder capacitor (as illustrated inFIG. 3).

The capacitor has a characteristic distance, denoted d. For example, fora parallel-plate capacitor, the characteristic distance d is thedistance separating the two plates. For a capacitor with interdigitatedelectrodes, the characteristic distance d is equal to λ/4, where λ isthe period of the interdigitated structure. Lastly, for acoaxial-cylinder capacitor, the characteristic distance d is the radialdistance between the two cylinders.

The excitation electrodes 1, 2 may have different forms, such as planarforms, cylindrical forms, interdigitated forms, etc.

The pair of excitation electrodes 1, 2 is advantageously covered with adielectric film 20. By way of non-limiting example, the dielectric film20 may be made from a dielectric selected from a polyimide, apolytetrafluoroethylene, and a photosensitive resin.

By way of non-limiting example, the excitation electrodes 1, 2 may bemade from a metal preferably selected from Cu, Ag, Au, and Al. However,the excitation electrodes 1, 2 may be made from a plastic (e.g., apolyphthalamide) into which carbon fibers have been incorporated inorder to make the excitation electrodes 1, 2 electrically conductive.

Ground Plane

The device advantageously comprises:

-   -   a dielectric layer 10, comprising a first surface and an        opposite second surface, the pair of excitation electrodes 1, 2        extending to the first surface of the dielectric layer 10; and    -   a counter-electrode, extending to the second surface of the        dielectric layer 10, and forming the ground plane PM.

Such a dielectric layer 10 allows the excitation electrodes 1, 2 and thecounter-electrode to be electrically insulated from each other so as toavoid short-circuiting them.

By way of non-limiting example, the counter-electrode may be a platemade from a metal. The metal is preferably selected from Cu, Ag, Au, andAl. However, the counter-electrode may be made from a plastic (e.g., apolyphthalamide) into which carbon fibers have been incorporated inorder to make the counter-electrode electrically conductive.

By way of non-limiting example, the dielectric layer may be made from adielectric selected from a polyimide and a polytetrafluoroethylene.

Electronic Circuit

The working frequency, at which the electronic circuit 3 applies anelectrical potential V+, V− to each excitation electrode 1, 2, is chosenso as to be lower than or equal to the cut-off frequency of at least oneof the phases P₁, P₂, P₃. As mentioned above, the working frequency mayset to 1 kHz to detect the presence of an aqueous phase P₁. Asillustrated in FIG. 1, the electronic circuit 3 may comprise anexcitation source (e.g., an AC voltage generator) for charging oneexcitation electrode 1 of the pair in order to apply thereto anelectrical potential V+. The other excitation electrode 2 of the pairmay be connected to the ground plane PM in order to apply thereto theelectrical potential V.

The electronic circuit 3 advantageously comprises a virtual ground 30connected to one excitation electrode 1, 2. The electronic circuit 3 isadvantageously configured to measure the transcapacitance between thepair of excitation electrodes 1, 2 using a three- or four-wire method.

The electronic circuit 3 advantageously comprises an operationalamplifier 31, which is used as an inverter and comprises:

-   -   a non-inverting input, connected to the ground plane PM; and    -   an inverting input, connected to an excitation electrode 2.        The operational amplifier 31 is employed in linear regime so as        to place the excitation electrode 2 connected to the inverting        input at ground potential virtually.

Control Electrode(s)

The distance at which is arranged the control electrode 4 of thecapacitor, which distance is denoted I, is chosen so that:

$0 < l < {100 \times d\frac{f_{c}}{f}\mspace{14mu} {and}\mspace{14mu} {preferably}\mspace{14mu} {so}\mspace{14mu} {that}}$$0 < l < {10 \times d\frac{f_{c}}{f}}$

where:

-   -   f_(c) is the cut-off frequency, and    -   f is the working frequency.

As illustrated in FIG. 4, the device advantageously comprises a set ofcontrol electrodes 4 that are arranged at different distances I₁, I₂, I₃from the capacitor, and that are intended to be inserted into themultiphase medium M. Each of the different distances (denoted I₁) atwhich are arranged the control electrodes 4 of the set advantageouslyrespects:

$0 < l_{i} < {100 \times d\frac{f_{c}}{f}\mspace{14mu} {and}\mspace{14mu} {preferably}\mspace{14mu} {so}\mspace{14mu} {that}}$$0 < l_{i} < {10 \times d\frac{f_{c}}{f}}$

As illustrated in FIG. 4, the control electrodes 4 of the set are spacedapart horizontally, this allowing the multiphase medium M to be studiedin this dimension.

As illustrated in FIG. 5, the device may comprise a set of controlelectrodes 4, which electrodes are arranged at the same distance l fromthe one or more capacitors, and spaced apart vertically so as to becoplanar, this allowing the multiphase medium M to be studied in thisdimension.

The control electrode 4 or the control electrodes 4 may take the form ofa grid, or the form of meanders. In the case of a control electrode 4taking the form of a grid, one advantageous effect thereof is to promotecontact with the conductive phase of the multiphase medium M, whetherthe phase be distributed surfacewise or volumewise, for example when theconductive phase is a foam or an emulsion. In the case of a controlelectrode 4 taking the form of meanders, the electrode mayadvantageously be planar, and parallel to the interdigitated excitationelectrodes 1, 2. The pitch of the meanders will possible be chosen so asto detect all or some of the conductive phase of the multiphase mediumM, when the conductive phase is distributed alternately (for example,when droplets that are separate from one another wet a surface of themeanders) in order to allow the fraction of the conductive phasecovering the meanders to be detected.

By way of non-limiting example, the control electrode 4 or the controlelectrodes 4 may be made from a metal, which is preferably selected fromCu, Ag, Au, and Al. However, the control electrode 4 or the controlelectrodes 4 may be made from a plastic (e.g., a polyphthalamide) intowhich carbon fibers have been incorporated in order to make the controlelectrodes 4 electrically conductive.

Switching Circuit

When the device comprises a set of control electrodes 4, the switchingcircuit 5 comprises one dedicated switch 50 for each control electrode4.

By way of non-limiting example, the switch 50 may be an on/off switch oran electrical relay.

Application to Phase Detection

The device, according to the present disclosure, may be a detector ofpresence of one of the phases P₁, P₂, P₃ provided that there is adifference between the transcapacitance measured by the electroniccircuit 3 between the pair of excitation electrodes 1, 2 when the switch50 is in the open state (denoted C_(off)), and when the switch 50 is inthe closed state (denoted C_(on)). The difference measured betweenC_(off) and C_(on) is indicative of the difference in dielectricresponse (in terms of electrical permittivity e) of the detected phasewhen the latter is excited by an electrical potential, whether thelatter is an exterior potential or not.

In practice, a detection threshold will be defined for the differencebetween C_(off) and C_(on), above which threshold the presence of thephase is ensured.

Application to Phase Quantification

Let the following be considered:

-   -   a multiphase medium M containing three stratified phase P₁, P₂,        P₃ that differ in their dielectric properties, and that have a        height h₁, h₂, h₃ in a vessel R of height H, respectively; and    -   that the phase P₁ has a differentiated dielectric response in        the presence of an exterior electrical potential.

Noting x₁=h₁/H; x₂=h₂/H; x₃=h₃/H

it is possible to establish the following equations:

x ₁ +x ₂ +x ₃=1

C _(off) =x ₁ C _(1,off) ^(H) +x ₂ C ₂ ^(H) +x ₃ C ₃ ^(H)

C _(on) =x ₁ C _(1,on) ^(H) +x ₂ C ₂ ^(H) +x ₃ C ₃ ^(H)

where:

-   -   C₂ ^(H) and C₃ ^(H) are the transcapacitances between the pair        of excitation electrodes 1, 2 when the vessel R is filled with a        phase P₂ and filled with a phase P₃, respectively, and    -   C_(1,on) ^(H) and C_(1,off) ^(H) are the transcapacitances        between the pair of excitation electrodes 1, 2 when the vessel R        is filled with a phase P₁, and when the switch 50 is in the        closed state and in the open state, respectively.

It is then possible to obtain the following relationships:

$x_{1} = \frac{C_{on} - C_{off}}{C_{1,{on}}^{H} - C_{1,{off}}^{H}}$$x_{2} = \frac{\left( {\left( {C_{off} - {x_{1}C_{1,{off}}^{H}}} \right) - {C_{3}^{H}\left( {1 - x_{1}} \right)}} \right)}{C_{2}^{H} - C_{3}^{H}}$$x_{3} = \frac{\left( {\left( {C_{on} - {x_{1}C_{1,{on}}^{H}}} \right) - {C_{2}^{H}\left( {1 - x_{1}} \right)}} \right)}{C_{3}^{H} - C_{2}^{H}}$

The value x₁ (and therefore h₁) is perfectly determined because:

-   -   C_(on) and C_(off) are values measured during the acquisition,        and    -   C_(1,on) ^(H) and C_(1,off) ^(H) are values measured by prior        calibration (see the following section).

In the same way, it is possible to determine x₂ (and therefore h₂) andx₃ (and therefore h₃) on the basis of the acquisition measurements andof the calibration measurements.

Calibration of the Device

If the device is to be used for phase quantification, it is necessary tocalibrate the device beforehand in order to determine the values ofC_(1,on) ^(H), C_(1,off) ^(H), C₂H, C₃ ^(H).

These calibrations may also be performed based on two other conditionsof known levels, or by similarity with another medium, or even bynumerical simulation.

It is possible to calibrate the device in-situ, using additionalcapacitive sensors (compensating capacitors) and techniques known tothose skilled in the art.

Installation

One subject of the present disclosure is an installation, comprising:

-   -   a vessel R containing a multiphase medium M; and    -   a device according to the present disclosure, the control        electrode 4 being inserted into the multiphase medium M.

The term “vessel” has a broad meaning and covers any means allowing themultiphase medium M to be contained.

The vessel R is advantageously electrically insulated from themultiphase medium M in order not to apply electrical potential to thecontrol electrode 4 (left floating by the switch 50 in the open state).The electrical potential thus remains set by the multiphase medium Mwhen the switch 50 is in the open state. When the vessel R is notelectrically insulated from the multiphase medium M (e.g., a vessel Rwith metal walls), then the capacitor and the control electrode 4 arearranged at a sufficiently large distance from the metal walls of thevessel R so that the vessel R does not influence the electricalpotential of the multiphase medium M.

The pair of excitation electrodes 1, 2 may be inserted into themultiphase medium M. By way of variant, the pair of excitationelectrodes 1, 2 may be placed on the exterior side of a dielectric wallserving to contain the multiphase medium M.

Examples of Embodiments

As illustrated in FIGS. 7 and 8, tests were performed on a multiphasemedium M containing an aqueous phase A, an oil phase B and an air phaseC in a vessel R consisting of a test-tube of a height H.

The capacitor was formed by a pair of interdigitated excitationelectrodes 1, 2. Each excitation electrode 1, 2 had a width of 250 μm.The inter-electrode distance was 250 μm. The capacitor had a height of300 mm. The capacitor extended over one face of a dielectric layer 10made of polyimide. The dielectric layer 10 had a thickness of 25 μm. Thepair of excitation electrodes 1, 2 was covered with a dielectric film 20made of polyimide. The dielectric film 20 had a thickness of 25 μm.

The working frequency was 1 kHz. The transcapacitance between the pairsof excitation electrodes 1, 2 was measured using an LCR meter.

The calibration to determine the values C_(A,on) ^(H), C_(A,off) ^(H)was carried out by filling a test-tube with water and by submerging thepair of excitation electrodes 1, 2 and the control electrode 4 in thetest-tube. The same protocol was observed for the oil.

In the case illustrated in FIG. 7, oil was gradually poured into atest-tube that initially contained only water. In the case illustratedin FIG. 8, water was gradually poured into a test-tube that initiallycontained only oil. In both cases it was observed that the fluid-levelvalues determined on the basis of the capacitive measurement of thedevice, according to the present disclosure, coincided with the fluidlevels observed by eye.

System for Taking Measurements with a Floating Device

As illustrated in FIGS. 9a to 9e , one subject of the present disclosureis a system for taking capacitive measurements in a multiphase medium M,comprising:

-   -   a floating device 6, intended to float in the multiphase medium        M; and    -   at least one device 7 according to the present disclosure, which        device is securely fastened to the floating device 6.

As illustrated in FIG. 9e , the floating device 6 may comprise aseparating wall 60 forming a barrier to the multiphase medium M, thewall 60 possessing an internal surface, and the device 7 for takingcapacitive measurements is mounted inside the wall 60, against theinternal surface.

The multiphase medium M may comprise three stratified phases comprising,in succession:

-   -   a liquid first phase P₁, which is electrically conductive at the        working frequency;    -   a liquid second phase P₂, which is liable to contain one or more        pollutants such as oils or hydrocarbons, and which is dielectric        at the working frequency; and    -   a gaseous third phase P₃, which is dielectric at the working        frequency.

The floating device 6 is arranged to float on the surface of the liquidsecond phase P₂. The floating device 6 may be a buoy. The floatingdevice 6 may comprise a number of floats 6 a, 6 b. The floating device 6may be of conical shape.

Of course, the electronic circuit 3 and the switching circuit 5 of thedevice 7 for taking capacitive measurements are seal-tight with respectto the multiphase medium M. The device 7 for taking capacitivemeasurements advantageously comprises a module 70 configured to transmitthe height of the liquid second phase P₂, the transmission possiblybeing performed via a wireless communication. The device 7 for takingcapacitive measurements is advantageously powered electrically:

-   -   by a battery, or    -   by a system for harvesting energy from the movement or        variations in temperature of the floating device 6, or    -   by a power source connected to a solar collector,        in order to make the operation of the device 7 for taking        capacitive measurements autonomous.

For an application for limiting the height of the liquid second phaseP₂, as illustrated in FIGS. 9a, 9c and 9d , the excitation electrodes 1,2 and the one or more control electrodes 4 of the device 7 for takingcapacitive measurements may be arranged to extend vertically, in contactwith the liquid first and second phases P₁, P₂, and to a depth slightlylarger than the height of the liquid second phase P₂.

For an application for detecting pollutants, as illustrated in FIG. 9b ,the excitation electrodes 1, 2 and the one or more control electrodes 4of the device 7 for taking capacitive measurements are advantageouslyarranged to lie entirely in the liquid second phase P₂, preferablysubstantially parallel to the free surface of the liquid second phaseP₂.

The invention is not limited to the described embodiments. Those skilledin the art will be able to consider technically workable combinationsthereof, and to substitute equivalents therefor.

1. A device for taking capacitive measurements in a multiphase medium,comprising: at least one pair of excitation electrodes, forming acapacitor; a ground plane; and an electronic circuit, arranged toelectrically connect the pair of excitation electrodes to the groundplane, and configured to: apply an electrical potential (V+, V−) to eachexcitation electrode at a working frequency, and measure atranscapacitance between the pair of excitation electrodes; wherein thedevice also comprises: at least one control electrode, arranged at adistance from the capacitor, and intended to be inserted into themultiphase medium; a switching circuit comprising a switch having anopen state and a closed state, in which states the switch electricallydisconnects and connects the control electrode from/to the ground plane,respectively, the switching circuit being configured to: apply, to thecontrol electrode, an electrical potential common to the ground plane,when the switch is in the closed state, and leave the electricalpotential of the control electrode floating, when the switch is in theopen state; and wherein the electronic circuit is configured to measurethe transcapacitance between the pair of excitation electrodes when theswitch is in the open state and when the switch is in the closed state.2. The device of claim 1, wherein: the multiphase medium comprises aphase containing species that are electrically conductive at the workingfrequency; the species possessing a cut-off frequency, below which thespecies equalize the electrical potential, left floating, in themultiphase medium, over the distance at which is arranged the controlelectrode of the capacitor; and the working frequency is chosen so as tobe lower than or equal to the cut-off frequency.
 3. The device of claim2, wherein: the capacitor has a characteristic distance; and thedistance at which is arranged the control electrode of the capacitor ischosen so that:$0 < l < {100 \times d\frac{f_{c}}{f}\mspace{14mu} {and}\mspace{14mu} {preferably}\mspace{14mu} {so}\mspace{14mu} {that}}$$0 < l < {10 \times d\frac{f_{c}}{f}}$ where: l is the characteristicdistance of the capacitor, d is the distance at which is arranged thecontrol electrode of the capacitor, f_(c) is the cut-off frequency, andf is the working frequency.
 4. The device of claim 1, further comprisinga set of control electrodes, of the at least one control electrode, thecontrol electrodes of the set being arranged at various distances fromthe capacitor and being intended to be inserted into the multiphasemedium, the switching circuit comprising one dedicated switch for eachof the control electrodes.
 5. The device of claim 1, wherein: theelectronic circuit comprises a virtual ground connected to an excitationelectrode, of the pair of excitation electrodes, and wherein theelectronic circuit is configured to measure the transcapacitance betweenthe pair of excitation electrodes using a three- or four-wire method. 6.The device of claim 5, wherein the electronic circuit further comprisesan operational amplifier used as an inverter, the operational amplifiercomprising: a non-inverting input, connected to the ground plane; aninverting input, connected to an excitation electrode.
 7. The device ofclaim 1, further comprising: a dielectric layer, comprising a firstsurface and an opposite second surface, the pair of excitationelectrodes extending to the first surface of the dielectric layer; and acounter-electrode, extending to the second surface of the dielectriclayer and forming the ground plane.
 8. The device of claim 1, whereinthe pair of excitation electrodes is covered with a dielectric film. 9.The device of claim 1, wherein the capacitor formed by the at least onepair of excitation electrodes is selected from a parallel-platecapacitor, a capacitor with interdigitated electrodes, and acoaxial-cylinder capacitor.
 10. An installation, comprising: a vesselcontaining a multiphase medium; and the device of claim 1, the controlelectrode being inserted into the multiphase medium.
 11. A system fortaking capacitive measurements in a multiphase medium, comprising: afloating device, intended to float in the multiphase medium; and atleast one of the device of claim 1, securely fastened to the floatingdevice.
 12. The system of claim 11, wherein: the floating devicecomprises a separating wall forming a barrier to the multiphase medium,the wall possessing an internal surface, and the device of claim 1 ismounted inside the wall, against the internal surface.